Wells are generally drilled into the ground to recover natural deposits of oil and gas or other minerals that are trapped in geological formations. To drill a well, a drill bit is connected on the lower end of an assembly of drill pipe sections that are connected end-to-end so as to form a “drill string.” The bit is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. A drilling fluid is pumped down through the drill string to the drill bit where it exits and carries drilled cuttings away from the bottom hole to the surface through the annulus between the drill string and the borehole wall.
In addition to the drill bit, the bottom hole assembly (“BHA”) commonly includes other tools, sensors, or equipment thereon used in the drilling process. Downhole tools may also be suspended in the wellbore on a wireline, which is lowered into the wellbore after the drilling process has completed or during interruptions in the drilling process when the drill string has been removed from the well.
Many of the tools, sensors, and other equipment used in downhole applications use electrical power in order to operate or actuate the device. Tools located on the drill string may be powered by a turbine or other motor through which the drilling fluid is circulated. However, when there is a lack of fluid circulation (or when the tool is located on a wireline), auxiliary power may be required. This auxiliary power may be in the form of a battery or supercapacitor that is attached to the downhole tool.
Downhole devices, including independent power sources, are configured to be able to withstand severe operating conditions (as compared to those devices used on the surface), principally higher temperatures and pressures than those experienced on the surface. High levels of vibration may also be encountered. In general, downhole pressure increases with depth into the formation, and to balance this pressure the hydrostatic pressure of the drilling fluid in the wellbore is increased commensurately. In addition to increased pressures, temperatures experienced downhole also generally increases with depth into the formation. Thus, downhole tools often operate in a high temperature environment where temperatures may exceed 125° C., which is higher than the normal operating range of commercial power sources (less than 80° C.).